What's the big deal?

It's a standard protocol for distributed computing. It's rides on top
of the HTTP protocol (yes, it's relatively "expensive", but it works) kind
of like CGI, except you get back standard, readable, platform appropriate
values. There are clients and servers for most of the major scripting
languages. You don't need to know anything about XML for it to work; XML
is the underlying communication protocol, but should be essentially
invisible.

Why XML-RPC for ASP

This package has risen to prominence lately because it is used in a
mission-critical production system.